The present invention relates to a device for controlling a shift in gear position in an automatic transmission of a motor vehicle.
Commonly, an automatic transmission has a plurality of gear positions, each providing a speed ratio peculiar to the particular gear positions, between a transmission input shaft and a transmission output shaft. Under the control of a control system, a command for one of the plurality of gear positions is made and the one gear position is established so as to produce an output torque sufficiently large enough to keep a motor vehile installed with the automatic transmission running. If there is a change in the running state of the motor vehicle, a change in gear position command, i.e., a shift command, takes place in the control system. After a delay (i.e., a response delay) after an instant when the change in command has taken place, a plurality of friction elements are put into action to initiate a shift. There is another delay (i.e., a shift delay) after the initiation of the shift until the completion thereof. During this transient period when the shift is being effected, a ratio of the revolution speed of the input shaft to that of the output shaft undergoes a change from a speed ratio peculiar to the old gear position to a speed ratio peculiar to a new gear position. If this change during this transient period is not smooth. substantial shocks are generated. However, since the shift is controlled in an open loop manner, it is next to impossible to always make the speed ratio change in a predetermined schedule tailored to the shift. Thus, it is desired to control this shift in a closed loop manner (i.e., feedback control of the shift). In order to realize this feedback control, an excellent detector for detecting or measuring a speed ratio of the revolution speed of the input shaft to that of the output shaft is needed.
Referring to a motor vehicle installed with an automatic transmission having an input shaft drivingly connected to an engine via a torque converter, a discussion will proceed hereinafter why substantial shocks are produced during a shift from a low gear position to a high gear position with an engine accelerator pedal kept depressed. One typical example is illustrated in FIG. 18 where there occurred at the instant t.sub.1 a change in command from a second gear position to a third gear position. Referring to FIG. 18, at the subsequent instant t.sub.2 after a response delay .DELTA.T.sub.1 from the instant t.sub.1, friction elements are put into action to initiate a shift, and the action of the friction elements is terminated to complete the shift at the subsequent instant t.sub.3 after a shift delay .DELTA.T.sub.2 from the instant t.sub.2. The speed ratio is subject to a change from a speed ratio R.sub.L (1.4, for example) peculiar to the second gear position to a speed ratio R.sub.H (1.0, for example) peculiar to a third gear position as illustrated by a fully drawn line during the time interval .DELTA.T.sub.2 where the action of the friction elements progresses. The revolution speed of the transmission input shaft, i.e., an input shaft revolution speed Ni, also decreases in response to the change in the speed ratio during this time interval .DELTA.T.sub.2 beginning with t.sub.2 and ending with t.sub.3 until it becomes equal to the revolution speed of the transmission output shaft, i.e., an output shaft revolution speed No. With the same opening degree of an engine throttle, an input torque Ti fed to the automatic transmission, which is generally equal in amount to an output torque of the engine Te, varies in inverse proportion to the above mentioned variation of the input shaft revolution speed Ni during the time interval .DELTA.T.sub.2. Theoretically, since it results from multiplying the input torque Ti with the speed ratio, an output torque To of the transmission stays substantially unchanged during the shift. However, the inertia of the engine works to resist the tendency of the input shaft decreasing its speed during this time interval .DELTA.T.sub.2, applying a torque Tm due to inertia to the transmission input shaft, causing an increase as shown by To' in the transmission output torque To during the shift, causing generation of substantial shocks.
Japanese patent application laid-open No. 58-207556 discloses a device for alleviating substantial shocks during a shift by causing a drop, in the engine output, from the normal level during the shift so as to suppress the above mentioned increase in the transmission output shaft. More particularly, according to this known device, the amount of delay in spark timing is increased during the shift so as to cause a drop in the engine output torque, thus suppressing the increase in the transmission output shaft. The initiation and termination of the above mentioned spark timing are brought into agreement with the initiation and termination of each shift in gear position taking place in the automatic transmission under the control of a timer circuit.
Japanese patent application laid-open No. 58-77138 discloses a method of alleviating shocks taking place during a shift in an automatic transmission. According to this known method, the output torque of an engine is subject to a temporal variation (i.e., a temporal drop/increase) during a shift. The temporal variation in the engine output torque is initiated at a predetermined timing with the instant when a change in command for gear position takes place.
Referring to a motor vehicle installed with a so-called lock-up type automatic transmission having an input shaft drivingly connected to an engine via a lock-up torque converter, i.e., a torque converter with a lock-up clutch, a discussion will proceed hereinafter how the lock-up clutch is released during a shift so as to alleviate shocks which would otherwise take place should the lock-up clutch be kept engaged during the shift.
Japanese patent application laid-open No. 56-127856 which has a U.S. counterpart, now U.S. Pat. No. 4,431,095 issued on Suga on Feb. 14, 1984 discloses a device for controlling a shift in a lock-up type automatic transmission. The operation of this known device is illustrated in FIG. 25 wherein a change in command from a second gear position to a third gear position occurred at the instant t.sub.1. Describing the operation of this known device referring to FIG. 25, a lock-up signal is subject to a change from ON level to OFF level at the subsequent instant t.sub.2 after a delay .DELTA.T.sub.1 from the instant t.sub.1. The OFF level of the lock-up signal causes the lock-up torque converter to assume its converter state after releasing its lock-up clutch. The OFF level of the lock-up signal is maintained during a time interval .DELTA.T.sub.2 beginning with the instant t.sub.2. After this time interval .DELTA.T.sub.2, the lock-up signal resumes ON level. In response to this temporal stay of the lock-up signal in OFF level, the lock-up torque converter starts effecting a shift from lock-up state to converter state immediately after the instant t.sub.2, but it does not resume lock-up state immediately after the change in the lock-up signal from OFF level to OFF level. The resumption to lock-up state is considerably delayed and starts at the instant t.sub.5 as illustrated. This characteristic is attributed to the construction of a lock-up control hydraulic circuit. At the subsequent instant t.sub.3 after a response delay .DELTA.T.sub.3 from the instant t.sub.1, friction elements are put into action to initiate a shift in gear position, and the shift is completed at the instant t.sub.4 after a shift delay .DELTA.T.sub.4. The speed ratio changes from a speed ratio R.sub.2 peculiar to the second gear position down to a speed ratio R.sub.3 peculiar to the third gear position. Since the response delay .DELTA.T.sub.3 is subject to a change owing to manufacturing variation from one product to another and/or oil temperature, the delay .DELTA.T.sub.1 is set as being shorter than the response delay .DELTA.T.sub.3 so as to allow a variation in the response delay, thus leaving a time interval from the instant t.sub.2 to t.sub.3 where the lock-up torque converter works as a torque converter even though the shift is not yet initiated. During this time interval from t.sub.2 to t.sub.3, the engine speed N.sub.E sharply increases up to a level N.sub.E1 that is considerably higher than a level N.sub.E2 which the engine speed would reach if the time interval from t.sub.2 to t.sub.3 were substantially zero (t.sub.2 =t.sub.3). Considering a torque due to inertia that result from multiplying the inertia of the engine with the engine speed, the torque due to inertia increases considerably during a time interval from the instant t.sub.3 to t.sub.4, i.e., during the shift, causing substantial shocks to take place.
From the preceding description, it will be recognized that the known devices are not satisfactory in alleviating shocks during a shift because they control the automatic transmission in an open loop manner during the shift.
The present invention aims at controlling a shift in an automatic transmission in a closed loop manner in order to alleviate shocks which would occur during the shift in an automatic transmission.
The present invention aims also at detecting or measuring an actual value in a speed ratio of a revolution speed of a transmission input shaft to that of a transmission output shaft during a shift in gear position in a transmission.
Japanese patent application laid-open No. 57-120752 discloses a method of detecting a gear position established in a motor vehicle installed with a transmission. According to this known method, a pulse train signal generated by a rotational angle sensor of a distributor, and a pulse train signal generated by a vehicle speed sensor for a speed meter are used. The pulse train signal of the rotational angle sensor has a frequency variable in proportion to the revolution speed of a transmission input shaft, and the pulse train signal has a frequency variable in proportion to the revolution speed of a transmission output shaft. These pulse train signals are processed to find the revolution speed of the transmission input shaft and that of the transmission output shaft, and then the former is divided by the latter to give a speed ratio of input to output. In finding the revolution speed of each of the shafts, pulses generated during a unit length of time are counted and the result is generated as the revolution speed. In order to detect the revolution speed with a good accuracy, the unit length of time has to be sufficiently long. Besides, since the number of pulses counted during the unit length of time are considerably small at low vehicle speed, the precision drops considerably. Thus, this known method is not suitable for detecting an actual value in the speed ratio during a shift in gear position in a transmission.